Blood is traditionally fractioned into cells and plasma by being centrifuged in systems of macroscopic dimensions. More recently, microfluidic techniques have also been developed.
In the field of microsystems, the technique in most widespread use is filtering. Filters are placed perpendicularly to the flow, with pores of dimensions that are optimized for retaining the particles, thereby recovering a fraction of the liquid phase. The main limitation of such techniques, when used with a biological solution, lies in the high deformability of certain cells (in particular red corpuscles in blood). The pores clog quickly, particularly with a solution that is highly concentrated, and the cells end up by lysing.
Another technique consists in performing separation by centrifuging at microfluidic scale, by injecting the suspension into a duct in the form of a spiral or a bend. Nevertheless, the secondary flows (Dean cells) that develop under such conditions tend to mix the particles that it is desired to separate from the liquid fraction. On this topic, reference can be made to the article by S. Ookawara, D. Street, and K. Ogawa entitled “Numerical study on development of particle concentration profiles in a curved microchannel”, Chem. Engineering Science 61 (2006), pp. 3714-3724.
One of the emerging techniques is extracting from a depleted zone. That technique is based on the fact that particles in suspension injected into a straight duct are subjected to non-uniform lateral migration as a result of shear forces; a particle-free zone thus appears at the edge of the channel followed by a superconcentrated ring surrounding a central zone in which concentration is uniform.
An application of that technique to extracting blood plasma is described in the article by M. Faivre, M. Abkarian, K. Bichraj, and H. Stone entitled “Geometrical focusing of cells in a microfluidic device: an approach to separate blood plasma”, Biorheology (2006) 43: pp. 147-159.
The main limitation of that technique is that any action exerted on the flow (e.g. to extract the plasma) disturbs it. Furthermore, the depleted zone phenomenon depends on flow conditions (liquid viscosity, rheological characteristics of the particles), which are conditions that vary greatly amongst patients and blood pathologies.